(.alpha.+.beta.)-alloys and .beta.-alloys containing Al, V, Zr, Sn, Cr, Mo, and the like have heretofore been known as high strength titanium alloys. In general, these conventional alloys have a tensile strength of at least 900 MPa, and there are few titanium alloys having a strength level between that of pure titanium and that of the conventional alloys, namely from about 700 to 900 MPa.
For example, Ti--6Al--4V alloy is a typical alloy of the (.alpha.+.beta.)-alloys, and has a tensile strength of 850 to 1,000 MPa and an elongation of 10 to 15% in an annealed state. There is Ti--3Al--2.5V alloy which has a strength level lower than the alloy mentioned above, and which has a tensile strength of 700 to 800 MPa and is excellent in ductility.
However, since these alloys contain V which is a high cost alloying element, they have the disadvantage that their cost is high.
Accordingly, the alloys mentioned below have been proposed in which V, a high cost alloying element, is replaced with Fe, a low cost element: Ti--5Al--2.5Fe alloy ("Titanium Science and Technology," Deutche Gesellshaft fur Metallkunde E.V. p1335 (1984)), and Ti--6Al--1.7Fe--0.1Si alloy and Ti--6.5Al--1.3Fe alloy (Advanced Material & Processes, p43 (1993)).
However, the above alloys which have been proposed contain a large amount of Al, and have high strength and low ductility at high temperature. The alloys have, therefore, poor hot workability compared with pure Ti. These alloys have the problem that the hot working cost is still high though the raw material cost is lowered by replacing V with Fe.
Accordingly, an alloy has been proposed which contains neither Al nor V and which utilizes O (oxygen) and N (nitrogen) as interstitial strengthening elements. For example, Japanese Patent Kokai Publication No. 61-159563 discloses a process for producing a pure Ti forged material having a tensile strength at the level of 80 kgf/mm.sup.2 class and an elongation of at least 20% which process comprises rough forging at high temperature including upsetting forging, finish forging, and heat treating at temperature of 500 to 700.degree. C. for up to 60 minutes. The process, however, requires complicated forging such as upsetting forging and heavy deformation, and it cannot be adopted in general.
Japanese Patent Kokai Publication No. 1-252747 discloses a high strength titanium alloy excellent in ductility which requires no specific forming, and which can be formed into products having various shapes such as sheets and rods by conventional rolling. The titanium alloy disclosed herein contains O, N and Fe as strengthening elements. The contents of these strengthening elements are defined as follows: the Fe content is from 0.1 to 0.8% by weight, and the oxygen equivalent value Q, which is defined to be equal to [O]+2.77[N]+0.1[Fe], is from 0.35 to 1.0. The N content is defined to be practically at least 0.05% by weight as disclosed in examples, and the titanium alloy is made to have fine microstructure in the (.alpha.+.beta.) dual and equiaxed phase or lamellar layers. As a result, the titanium alloy has a tensile strength of at least 65 kgf/mm.sup.2.
The disclosed titanium alloy attains a tensile strength of at least 65 kgf/mm.sup.2 and an elongation of at least 20% by solid solution strengthening with O and N, and by microstructural grain refining effects obtained by utilizing an Fe content higher than that of pure titanium, and it attains a tensile strength of at least 85 kgf/mm.sup.2 particularly when Q.gtoreq.0.6.
However, as shown in FIGS. 1 and 2 in the patent publication, the titanium alloy has a tensile strength of up to 95 kgf/mm.sup.2 when Q.ltoreq.0.8, though it has an elongation of at least 15%. Moreover, though the titanium alloy has a tensile strength as high as from 95 to 115 kgf/mm.sup.2 when Q=0.8 to 1.0, it has an elongation as low as up to 15%.
As described above, the titanium alloy does not always have both a high strength and a high ductility at the same time. Accordingly, a further development of a titanium alloy having both a high strength and a high ductility is desired.
Furthermore, although the alloy requires a N content as high as at least 0.05% by weight, the addition of such a large amount of N is extremely difficult in the production of the alloy by melting. Control of the addition amount is also difficult.
That is, since melting titanium is conducted in vacuum or in an inert gas atmosphere at low pressure, introducing nitrogen using a nitrogen gas is almost impossible during melting. Nitrogen, therefore, must be introduced in the form of a nitrogen-containing solid. To avoid a contamination with impurities which exert adverse effects on the properties of titanium, the addition of nitrogen-containing titanium is preferred. To obtain such a high nitrogen content as mentioned above, a technique such as addition of titanium containing a large amount of nitrogen becomes necessary. As a result, a compound such as TiN having a very high melting point of 3,290.degree. C., and likely to form an undissolved portion, may form. Such undissolved TiN, etc. may remain as nitrogen-rich inclusions in the titanium alloy, and it may form a fatal defect such as the starting point of a fatigue failure. Moreover, since nitrogen is a gas component, the introduced nitrogen tends to evaporate even when the nitrogen is introduced in the form of a nitrogen-containing solid, and control of the nitrogen content is difficult.